1. Field of the Invention
The present invention relates to improved thermoelectrics for producing heat and/or cold conditions with greater efficiency.
2. Description of the Related Art
Thermoelectric devices (TEs) utilize the properties of certain materials to develop a thermal gradient across the material in the presence of current flow. Conventional thermoelectric devices utilize P-type and N-type semiconductors as the thermoelectric material within the device. These are physically and electrically configured in such a manner that they provide cooling or heating. Some fundamental equations, theories, studies, test methods and data related to TEs for cooling and heating are described in H. J. Goldsmid, Electronic Refrigeration, Pion Ltd., 207 Brondesbury Park, London, NW2 5JN, England (1986). The most common configuration used in thermoelectric devices today is illustrated in FIG. 1. Generally, P-type and N-type thermoelectric elements 102 are arrayed in a rectangular assembly 100 between two substrates 104. A current, I, passes through both element types. The elements are connected in series via copper shunts 106 soldered to the ends of the elements 102. A DC voltage 108, when applied, creates a temperature gradient across the TE elements. TE's are commonly used to cool liquids, gases and objects. FIG. 2 for flow and FIG. 3 for an article illustrate general diagrams of systems using the TE assembly 100 of FIG. 1.
The basic equations for TE devices in the most common form are as follows:
                              q          c                =                              α            ⁢                                                  ⁢                          IT              c                                -                                    1              2                        ⁢                          I              2                        ⁢            R                    -                      K            ⁢                                                  ⁢            Δ            ⁢                                                  ⁢            T                                              (        1        )                                          q                      i            ⁢                                                  ⁢            n                          =                              α            ⁢                                                  ⁢            I            ⁢                                                  ⁢            Δ            ⁢                                                  ⁢            T                    +                                    I              2                        ⁢            R                                              (        2        )                                          q          h                =                              α            ⁢                                                  ⁢                          IT              h                                +                                    1              2                        ⁢                          I              2                        ⁢            R                    -                      K            ⁢                                                  ⁢            Δ            ⁢                                                  ⁢            T                                              (        3        )            where qc is the cooling rate (heat content removal rate from the cold side), qin is the power input to the system, and qh is the heat output of the system, wherein:
α=Seebeck Coefficient
I=Current Flow
Tc=Cold side absolute temperature
Th=Hot side absolute temperature
R=Electrical resistance
K=Thermal conductance
Herein α, R and K are assumed constant, or suitably averaged values over the appropriate temperature ranges.
Under steady state conditions the energy in and out balances:qc+qin=qh  (4)Further, to analyze performance in the terms used within the refrigeration and heating industries, the following definitions are needed:
                    β        =                                            q              c                                      q                              i                ⁢                                                                  ⁢                n                                              =                      Cooling            ⁢                                                  ⁢            Coefficient            ⁢                                                  ⁢            of            ⁢                                                  ⁢                          Performance              ⁡                              (                                  C                  ⁢                                                                          ⁢                  O                  ⁢                                                                          ⁢                  P                                )                                                                        (        5        )                                          γ          =                                                    q                h                                            q                                  i                  ⁢                                                                          ⁢                  n                                                      =                          Heating              ⁢                                                          ⁢              C              ⁢                                                          ⁢              O              ⁢                                                          ⁢              P                                      ⁢                                  ⁢                              From            ⁢                                                  ⁢                          (              4              )                                ;                                    (        6        )                                                                    q              c                                      q                              i                ⁢                                                                  ⁢                n                                              +                                    q                              i                ⁢                                                                  ⁢                n                                                    q                              i                ⁢                                                                  ⁢                n                                                    =                              q            h                                q                          i              ⁢                                                          ⁢              n                                                          (        7        )                                          β          +          1                =        γ                            (        8        )            So β and γ are closely connected, and γ is always greater than β by unity.
If these equations are manipulated appropriately, conditions can be found under which either β or γ are maximum or qc or qh are maximum.
If β maximum is designated by βm, and the COP for qc maximum by βc the results are as follows:
                              β          m                =                                            T              c                                      Δ              ⁢                                                          ⁢                              T                c                                              ⁢                      (                                                                                1                    +                                          ZT                      m                                                                      -                                                      T                    h                                                        T                    c                                                                                                                    1                    +                                          ZT                      m                                                                      +                1                                      )                                              (        9        )                                                      β            c                    =                      (                                                                                1                    2                                    ⁢                                      ZT                    c                    2                                                  -                                  Δ                  ⁢                                                                          ⁢                  T                                                                              ZT                  c                                ⁢                                  T                  h                                                      )                          ⁢                                  ⁢                  where          ;                                    (        10        )                                Z        =                                            α              2                        RK                    =                                                                      α                  2                                ⁢                ρ                            λ                        =                          Figure              ⁢                                                          ⁢              of              ⁢                                                          ⁢              Merit                                                          (        11        )                                          T          m                =                                            T              c                        +                          T              h                                2                                    (        12        )                                R        =                  ρ          ×          length          ⁢                      /                    ⁢          area                                    (        13        )                                K        =                  λ          ×          area          ⁢                      /                    ⁢          length                                    (        14        )                                          λ          =                      Material            ⁢                                                  ⁢            Thermal            ⁢                                                  ⁢            Conductivity                          ;        and                            (        15        )                                ρ        =                  Material          ⁢                                          ⁢          Electrical          ⁢                                          ⁢          Resistivity                                    (        16        )            
βm and βc depend only on Z, Tc and Th. Thus, Z is named the figure of merit and is basic parameter that characterizes the performance of TE systems. The magnitude of Z governs thermoelectric performance in the geometry of FIG. 1, and in most all other geometries and usages of thermoelectrics today.
For today's materials, thermoelectric devices have certain aerospace and some commercial uses. However, usages are limited, because system efficiencies are too low to compete with those of most refrigeration systems employing freon-like fluids (such as those used in refrigerators, car HVAC systems, building HVAC systems, home air conditioners and the like).
The limitation becomes apparent when the maximum thermoelectric efficiency from Equation 9 is compared with Cm, the Carnot cycle efficiency (the theoretical maximum system efficiency for any cooling system);
                                                        β              m                                      C              m                                =                                                                                          T                    c                                                        Δ                    ⁢                                                                                  ⁢                    T                                                  ⁢                                  (                                                                                                              1                          +                                                      ZT                            m                                                                                              -                                                                        T                          h                                                                          T                          c                                                                                                                                                              1                          +                                                      ZT                            m                                                                                              +                      1                                                        )                                                                              T                  c                                                  Δ                  ⁢                                                                          ⁢                  T                                                      =                          (                                                                                          1                      +                                              ZT                        m                                                                              -                                                            T                      h                                                              T                      c                                                                                                                                  1                      +                                              ZT                        m                                                                              +                  1                                            )                                      ⁢                                  ⁢        Note        ,                              as            ⁢                                                  ⁢            a            ⁢                                                  ⁢            check            ⁢                                                  ⁢            if            ⁢                                                  ⁢            Z                    →          ∞                ,                  β          →                                    C              m                        .                                              (        17        )            
Several commercial materials have a ZTA approaching 1 over some narrow temperature range, but ZTA is limited to unity in present commercial materials. Typical values of Z as a function of temperature are illustrated in FIG. 4. Some experimental materials exhibit ZTA=2 to 4, but these are not in production. Generally, as better materials may become commercially available, they do not obviate the benefits of the present inventions.
Several configurations for thermoelectric devices are in current use in applications where benefits from other qualities of TEs outweigh their low efficiency. Examples of uses are in automobile seat cooling systems, portable coolers and refrigerators, liquid cooler/heater systems for scientific applications, the cooling of electronics and fiber optic systems and for cooling of infrared sensing system.
All of these commercial devices have in common that the heat transport within the device is completely constrained by the material properties of the TE elements. In sum, in conventional devices, conditions can be represented by the diagram in FIG. 5. FIG. 5 depicts a thermoelectric heat exchanger 500 containing a thermoelectric device 501 sandwiched between a cold side heat exchanger 502 at temperature TC and a hot side heat exchanger 503 at temperature TH. Fluid, 504 at ambient temperature TA passes through the heat exchangers 502 and 503. The heat exchangers 502 and 503 are in good thermal contact with the cold side 505 and hot side 506 of the TE 501 respectively. When a DC current from a power source (not shown) of the proper polarity is applied to the TE device 501 and fluid 504 is pumped from right to left through the heat exchangers, the fluid 504 is cooled to TC and heated to TH. The exiting fluids 507 and 508 are assumed to be at TC and TH respectively as are the heat exchangers 502 and 503 and the TE device's surfaces 505 and 506. The temperature difference across the TE is ΔT.